Equipment and methods that create and utilize mechanical foam to extinguish fires are known. In particular, fire fighting equipment and methods that utilize foam generated mechanically in a foaming chamber are taught in U.S. Pat. Nos. 4,828,038, 4,497,442 and 5,167,285, which are assigned to the same assignee as the present invention.
In mechanical foam fire fighting equipment, a liquid, such as water, and a foam concentrate, such as the "AFFF" product of Minnesota Mining and Manufacturing Co., are typically supplied to a foam making assembly. The foam making assembly contains a foaming chamber for receiving the liquid and the foam concentrate, either separately or together.
Typically, the liquid is delivered under pressure. The foam concentrate may also be delivered under pressure. The foam concentrate may be educted into the assembly through eductor means supported and disposed within the foam making assembly, as known in the art, or the concentrate might be pumped or gravity fed to the assembly. The foam concentrate and liquid may be mixed, partially or totally, prior to supply to the assembly.
Air and/or ambient vapors in the atmosphere are inducted into the foam chamber according to the teachings of present art mechanical foam equipment. What is referred to as "mechanical foam" in the trade is sometimes also referred to as "air foam". Usually the air or ambient atmospheric vapors are inducted into the foaming chamber subsequent to, or at least simultaneously with, the supply of the mixture of the liquid and foam concentrate to the chamber. The air may also be supplied under pressure.
The foam making assembly may comprise a fire fighting nozzle that throws the foam generated to the fire. Alternately, the foam may be delivered from the assembly to the fire through discharge tubing or piping.
A mechanical foam making assembly includes a foaming chamber area appropriate for the mechanical formation of suitable bubbles from the concentrate, the liquid and the air. The mixing takes place as a result of the turbulence created in the chamber with the moving fluids. The turbulence in the chamber area aerates the liquid and concentrate into foam. The foam is then discharged from an outlet end of the mixing chamber area.
It should be understood that the primary bubbles of the foam are formed in the foaming chamber area. Depending upon the equipment this area is more or less defined by the physical structural walls of the assembly.
A subtle problem has been discovered associated with present art mechanical foam equipment. The problem has been encountered, in fact, utilizing equipment substantially as described in U.S. Pat. No. 4,828,038, and in particular as illustrated in FIG. 5 of that patent.
In the combustion of a large tank of flammable liquid, to discuss a key example, it is common for mechanical foam to be supplied to the tank by piping the foam to an inlet at a low level. The level is above any anticipated water level but below substantially all of the flammable fluid. According to the design of the equipment and technique, the foam, so injected, rises through the liquid contents of the tank to the surface. Upon reaching the surface, the foam isolates the burning contents from its necessary oxygen source, thereby choking off the fire.
This isolation and choking effect does not last for an unlimited period of time. The "25% drain time" of a particular foam is defined as the amount of time required for 25% of the bubbles comprising the foam to burst and form water. After the "25% drain time" period, it is recognized that a significant amount of the blanketing capacity of the foam is lost. Because of this loss, techniques are taught to attempt to extend the "25% drain time" of various foams in a variety of fire fighting situations. Nonetheless, the "drain time" remains a factor requiring the constant supply of new foam to the fire.
It is now appreciated that there is a potentially significant further effect from the bursting of the foam bubbles on the fire, in addition to the loss of the foam blanket and the formation of water. This effect arises from the freeing of the air or atmospheric vapors that are entrained in the formed bubbles.
During a recent extinguishment of a fire in a large flammable liquid storage tank having a floating roof, it is believed that a countervailing effect was experienced from the oxygen released from the entrained air. The oxygen released from the air or atmosphere in the bubbles under the floating roof appeared to feed the fire. The supply of oxygen raised the possibility that the enclosed space under the roof might reach an explosive range.
The present invention solves the above problem. The present invention discloses an "inert mechanical foam", useful not only in applications such as the above referenced flammable liquid tank fire, but also in many other situations. One such application might involve the use in an enclosed or semi-enclosed space such as a fuselage of a burning airplane or a room or compartment within a burning building or ship. An inert foam would even have some usefulness on fires exposed to the atmosphere.
"Inert mechanical foam" is used herein to mean a mechanical foam whose bubbles are created through the agitation of a foam concentrate, a liquid and an inert gas. An inert gas is supplied in lieu of, or at least predominantly in lieu of, utilizing the standard air or prevalent ambient atmospheric vapors as taught by the prior art. "Inert gas" refers to an inert material that is generally gaseous at ambient temperature and pressure. This inert gas, of course, could be liquified for delivering, supply and/or storage purposes.
An inert mechanical foam, when its bubbles burst, would not serve to feed a fire additional oxygen but would rather provide an additional choking effect.
A further aspect of the present invention is that the means for generating an inert gas for use in producing an inert mechanical foam is commonly at hand at most fire scenes. Most fire fighting equipment utilize an engine, such as a diesel or a propane engine, as a means for pumping or at least for transportation purposes. Engines can be regarded, in effect, as inert gas generators. A primary product of most combustion engines is the inert gas CO.sub.2. Calculations indicate that the size of most engines associated with fire fighting equipment is sufficient to generate the inert gas needed to aerate the mechanical foam produced by the equipment. The amount of undesirable by-products of the combustion of the engine is relatively low, considering the circumstances, and even those can be filtered. The engine itself can further be used to power a blower to propel or pressure the exhaust gas to the assembly. The exhaust gas could be cooled, as with water, if such appeared necessary.
Commercially available inert gas generators are also usually found onboard ship. It is known to use gas from such generators or shipboard flue gas to perform certain tank cleaning functions on board. Such inert gas generators or sources of shipboard flue gas could also be used as the supply of inert gas for producing the inert mechanical foam of the present invention.
The above invention relates to equipment for producing, and methods of use for, what is commonly called in the trade "mechanical foam". This is a foam created by mechanical agitation. It comprises the primary, if not sole, fire fighting foam used today. "Mechanical foam" is sometimes also referred to as "air foam".
A different form of foam has been known historically in the field. This foam is called "chemical foam" and is created by a chemical reaction, generally between an acid and a base. Chemical foams have been known in both dry and aqueous forms. Both forms use the same chemicals: part A (acidic) aluminum sulfate and part B (basic) sodium bicarbonate.
Proteinaceous foam stabilizers are typically added to form the bubbles.
"Chemical foam" happens to produce an inert foam. This foam, however, has not been used for many years in the fire fighting industry for a variety of reasons. The utilization is and has been limited by the difficulties involved in the storage of sufficient chemicals, in the production of foam in sufficient quantities and in the transportation and delivery of the chemical foam to the fire. Chemical foam does not play a significant role in present fire fighting techniques, if indeed it is used at all.